(a) Field of the Invention
This invention relates generally to signal detecting systems and more particularly to receivers employing an auxiliary detector for detecting the arrival of an expected transmitted signal by correlating the phase of a local phase-coded signal, produced by a local replica generator, with the phase of the transmitted signal.
(b) Description of the Prior Art
In spacecraft receivers as well as in many other signal receiving environments, the signal-to-noise (S/N) ratio is unusually low. Accordingly, special care must be taken to design accurate and reliable signal receivers for use in such environments. Most spacecraft receivers must remain operative for long periods of time without requiring adjustments or repair. For example, when such a receiver is used in a Jupiter probe, it will not be returned to earth for service and must function for a long time under extremely severe signal transmission conditions.
On the other hand, in the shuttle communication and tracking equipment (SCTE), the receiver will come back with the shuttle. It is used for communicating with the earth and with other spacecrafts. Usually the power of the transmitted signal which arrives to the receiver is very low: large distances between the earth transmitter and the spacecraft receiver exist; large levels noise distort the transmitted signal, for example, when the spacescraft's antenna is pointing toward the sun. Thus, in such systems, the S/N ratio may become extremely low, say on the order of -22.5 db.
Auxiliary networks have been proposed for improving the reception of low-power signals by conventional receivers in an environment producing a low S/N ratio. Typically, known receivers include a local replica code signal generator that produces a signal that is phase modulated with a prescribed replica code identical to the code used on ground for modulating the transmitted signal. The transmitted signal typically carries a pseudo-random-noise (PN) serial code. It was found that the PN code provides the best signal characteristics for this type of signal reception.
It is an object of one such known auxiliary network to act as a detector to detect the arrival of the expected code by adjusting the phase of the local replica code until it becomes in full alignment with the phase of the incoming code, and then to maintain synchronism of the local generator with the code on the received signal. To achieve this object, the known detector comprises a "reference" channel in parallel with a "signal" channel. The circuits in the two channels are fully matched for their electrical characteristics. The incoming PN code is applied simultaneously to the reference and signal channels.
The reference channel demodulates or correlates the incoming PN code with a local, completely incorrect, demodulating signal which has a code pattern totally distinct from the pattern of the incoming PN code carried by the transmitted signal.
The signal channel, on the other hand, demodulates the incoming PN code with the local, replica PN code. When the phase of the replica PN code is adjusted to become aligned with the phase of the incoming PN code, a substantial increase in the output level of the signal channel occurs, as compared to its output level produced when the replica and incoming codes are in phase error, or when the incoming signal has an incorrect code.
The output levels from the two parallel channels are subtracted in a summer to provide a difference signal which, if it exceeds a threshold level, is applied to the logic section of the receiver, commanding it to stop the search of the incoming PN code by the detector and to start tracking and detecting the incoming signal. The detector, therefore, serves as a threshold detector for the receiver.
Each channel of such a detector includes a mixer, a band-pass filter (BPF), a radio-frequency (RF) amplifier, an amplitude detector, and a low-pass filter (LPF) amplifier.
If the signal and reference channels had their circuits perfectly matched and if their electric characteristics did not change with time, use, and temperature, such detector would operate satisfactorily. However, the channels' electric characteristics do change with time, use, temperature, and with other environmental conditions. Therefore, the reference and signal channels develop with time different phase and gain characteristics. As a result, relatively expensive components and circuits have to be used to minimize such changes. Also, temperature-compensating networks have to be employed at least for the amplifiers in both the signal and reference channels. But even the temperature-compensating networks themselves change their electric characteristics with aging. Additionally, the filters, such as band-pass filters, require precise components so as to obtain a frequency response that remains substantially constant over the useful life of the receiver.
The need for the reference channel in the known threshold detector stems from the need to allow weak incoming signals to be detected without appreciably distorting strong signal reception. Using two channels it is possible to detect the same noise in both channels and to thereby produce noise cancellation by substraction. When the output of the reference channel is subtracted from the output of the signal channel, there is obtained a difference whose sign gives a clear indication of the presence or "acquisition" of the incoming PN code, and whose amplitude corresponds to the degree of correlation between the incoming PN code with the local PN code. Thus, the reference channel is used in accordance with the prior art teachings for calibrating the signal channel. But the effectiveness of such calibration changes with aging.
Another major drawback of the two-channel threshold detector is that it requires duplication of relatively expensive and large circuits and components used for each channel and for the temperature-compensating networks.
Accordingly, it is a general object of this invention to provide a new and improved threshold detector which requires a minimum of circuits thereby affording economy in cost and packaging.
The general object of the invention is accomplished by providing a single channel and by time-sharing the single channel with the aid of relatively inexpensive, small and generally available digital components.